When you looked at the flashlight beam, you saw tiny particles (very tiny pieces) floating through the air. The Earth's air is not just pure gas. It also contains particles of dirt, sand, cloth, metal, wood, plastic, ash, hair, and other materials. When the air becomes full of manufactured particles, such as soot, ash, or chemicals, we say it is polluted. Polluted air can be harmful to people and pets. That's one reason why spending time outdoors, opening windows, and having lots of indoor plants is good for people.